Japanese Patent Laid-Open Publication No. 2001-20975 discloses, in a four-wheel-drive vehicle, a torque transmission apparatus which transmits a motor torque from the side of main driving wheels (i.e. front wheels) to the side of sub-driving wheels (i.e. rear wheels) through a coupling.
In this coupling, oil is closely sealed in a rotating casing to lubricate sliding frictional parts, such as clutches and bearings, accommodated in the casing. With this arrangement, the coupling is ensured in its performance.